1. Field of the Invention
The present invention relates to a photoelectric converter such as an imaging apparatus employing photoelectric conversion devices such as photodiodes.
2. Description of Related Art
An imaging apparatus has a number of pixels formed in an imaging area (a light receiving portion) thereof receive light signals from a subject, photoelectrically converts the light signals into signal charges, transfers the converted electric signals, thereby imaging the subject.
FIG. 7 shows a CCD imaging apparatus as an example of an imaging apparatus.
Light incident on a photodiode 1 is photoelectrically converted into a signal charge, the signal charge is read out to a vertical CCD portion 3 via a readout gate 12A, the signal charge is transferred by supplying transfer clocks of, e.g. xcfx86V1 to xcfx86V4 to the vertical CCD 3, and the signal charge is directed to an output portion in a horizontal CCD portion to which transfer clocks of, e.g. xcfx86H1 and xcfx86H2 are supplied, where charge voltage conversion and the like are performed to produce an image signal.
By the way, recent imaging apparatuses, particularly a CCD (charge coupled device) imaging apparatus and the like, tend to have more compact unit cells and increase the amount of charges stored in a sensor to improve sensitivity.
For this reason, it is becoming difficult to read out charges into a vertical CCD, which is a charge transfer portion, from a photodiode comprising a light receiving portion (pixel portion) via a gate portion.
Particularly, as shown in FIGS. 4A and 4B, in the case of an imaging apparatus in which the horizontal size of a unit cell comprising a photodiode portion 1 and a gate portion 2 is longer than the vertical size, it is more difficult to read out charges into a vertical CCD portion 3. FIG. 4A shows a schematic layout of the unit cell from which the vertical CCD portion is excluded, and FIG. 4B shows a schematic layout of the unit cell including the vertical CCD portion (hereinafter the same is also true of other drawings). L2-1 or W1 in the drawings is the width of a readout gate 2A and L1-1 is the width of a gate 2B which is not related to readout. The reference numeral 4 designates a channel stopper for separating unit cells.
To solve such a problem, the following two measures are taken. One is, as shown in FIGS. 5A and 5B, to enlarge the readout gate width (gate length in a direction orthogonal to a carrier travel direction: the same is true of the following descriptions) L2-2 or W2 of a readout gate portion 2A (indicated by white background or oblique lines) of the gate portion 2. The other is, as shown in FIGS. 6A and 6B, to reduce a readout gate length (gate length in a carrier travel direction: the same is true of the following descriptions) 11-3.
However, in the layouts of FIGS. 5A and 5B, a ratio between the two gates of the vertical CCD portion 3, that is, a gate 2A to read out charges and a gate 2B which is not related to readout, is different. Consequently, the charge capacity of the vertical CCD portion is liable to become small or charges are liable to be unsuccessfully transferred.
In the layouts of FIGS. 6A and 6B, the gate 2 itself fails to play the role of a barrier during off operation, with the result that charges leak from the photodiode portion 1 to the vertical CCD portion 3 or light passes between a silicon film and a shielding film (both are not shown) below the gates, and photoelectric conversion may occur within the vertical CCD portion 3.
The present invention has been made in view of the above situation, and its object is to provide a photoelectric converter such as an imaging apparatus which smoothly reads out charges without influencing the charge capacity and charge transfers of a charge transfer portion and can reduce power consumption by reducing a readout voltage.
Specifically, a photoelectric converter of the present invention comprises a light receiving portion, a gate portion, and a charge transfer portion, wherein the gate width of the gate portion is wider at the light receiving portion side than at the transfer portion side.
Thus making a gate width wider at a light receiving portion side than at a charge transfer portion side helps to enlarge an area of an electric field applied to the gate in an area to admit charges from the light receiving portion (accordingly, the electric field of the gate area can be intensified), with the result that charges can be easily read, and if a readout capacity is the same, a charge readout voltage can be reduced to reduce power consumption. Yet, since such an effect can be achieved without changing a gate width at the charge transfer portion side, no influence is exerted on the charge capacity and charge transfers of the charge transfer portion and other characteristics also do not deteriorate.